From October 2012 the description here came from Pascal in France who had a st5742. (t5740 with no flash
and no operating system - see
HP's naming system).
He had found a relatively cheap st5742 from the US to replace his t5700 which was acting as a low power
home server. You'll find a description of what he did under the mods tab.

Finally [Feb 2015] I've got my hands on a t5740, albeit with no flash memory, no bottom cover and
with no stand but it was working and has enabled me to fill out this entry to match the others.

Specifications

Processor

Type Speed

Intel Atom N2801.66GHz

Memory

Flash RAM

2GB2GB (max 8GB)

Video

Chip Max resolution
Standard Widescreen

Intel GL40

2048 x 1536 32-bit colour1920 x 1200

Ports

Network USB Serial
Parallel PS/2

10/100/10008 x USB2.01none2

Power

Plug Supply
Off Idle Running

Coax 4.8mm/1.7mm19V 3.42A1W10W12W

Dimensions

H x W x D (mm)

267 x 44 x 222 with Stand

The operating system is Windows 7.

From HP's datasheet:

Intel GL40 graphics with support for two high-resolution monitors up to 2048 x 1536, or up to 32 bit color depth. Output through VGA and DisplayPort connectors. Optional adapters available for DisplayPort to VGA or DVI-D connections.

Power Supply

The power supply is a laptop-style 19V one with a coax connector. The outer diameter of the plug is 4.8mm.
I believe the inner diameter to be 1.7mm but it's not the easiest thing to measure the diameter of
the recessed pin in the socket, it might be 1.4mm. When I searched on line for 't5740 PSU' I was surprised to find
a number of offerings claiming to be replacements that were fitted with 5.5mm/2.5mm plugs. I can assure you
anything with those dimensions does not fit my t5740!

CPU

The CPU is an Intel Atom N280 CPU, a single core 32-bit CPU but with Hyperthreading.
For those to whom it matters here is some detail from Linux's /proc/cpuinfo

Expansion

Click on the image for a larger version.

Flash: As mine came without any flash fitted I'm not 100% certain what type of flash is
included. The board offers two ways of adding the flash - a 44-pin IDE connector or a SATA socket.
The SATA socket is one of the combined power and data sockets and is to the SATA II standard.

However I've had an email from Christian who tried to use a 3.5" disk with the t5740
and found it wasn't recognised and actually didn't spin up.

"...so I've checked the pins from the power connector with a multimeter and found
out that there is no power provided on the three 12V pins - not on the Combo-Connector
nor on the unequipped connector near the 44pin IDE connector.
The 5V and 3.3V pins provide the correct power. It works with 2.5" HDDs
because they usually don't use 12V, only 5V (some older ones also 3.3V)."

RAM: The motherboard has two sockets for SODIMMs, one on the top of the board and one on the bottom.
The one on the bottom is accessible by removing a small metal panel - you don't have to take the motherboard
out of the enclosure. The datasheet mentions '1 or 2 GB DDR3/667MHz system memory'. In my model there was
a single 1GB Hynix SODIMM fitted, PC3-10600S, part # HMT112S6TFR8C-H9. The datasheets put the maximum memory
supported as 8GB, i.e. 2 x 4GB modules. I've successfully fitted a 4GB Elpida part (2Rx8 PC3-10600S).

I also found the following 2GB parts I had to hand worked:

Hynix 2GB 1Rx8 PC3-10600S-9-10-B1 (HTM325S6BFR8C-H9)

Samsung 2GB 2Rx8 PC3-10600S-09-10-F2 (M471B5673EH1-CH9)

Micron Technology 2GB 1Rx8 PC3-10600S-9-10-B1 (MT8JSF25664HZ-1G4D1)

I also had a 4GB 1Rx8 PC3L-12800S part from Nanya
- (NT4GC64C88B1NS-DI) - that didn't work which made suspect that it didn't support 'PC3L' parts.
However I had an email from Parker Reed who was happily using a 2GB 1Rx16 PC3L-12800S part from
Samsung (M471B5674QH0-YK0).

NB:The N280 is a 32-bit processor and so can only directly address 4GB. It does support PAE
and so one imagines it could physically use more memory if you're running with a PAE kernel. However
reports I've seen so far and my own tests (see Linux tab) indicate that,
despite the BIOS showing that 4GB, 6GB or 8GB of RAM is fitted, nobody has been able to get their OS to see
more than the typical ~3GB maximum of a 32-bit CPU. If you know better then please email me using the
link at the bottom of the page.

At the moment it looks like there is no benefit in fitting more than 3GB of RAM.

USB: Like a lot of the HP thin clients of this era the t5740 includes two 'secure' USB
sockets within the housing. There are also two on the front panel and four sockets on the rear panel.

PCIe x4: There is a PCIe connector on the board adjacent to the SATA socket.

PCI: There is what looks like a PCI slot. However this is a non-standard connector that is
used with an expansion housing that doubles the width of the thin client. The words on the expansion Module
mention gaining a x4/PCI slot and a parallel and second serial port. Obvious connections to the latter
must pass over this interface.
An email I received about the t610 showed that in some models
at least the pin-out of the socket starts off as a genuine PCIe socket. So it is possible that pins 1-32
offer a standard PCIe x4 socket in a similar way to the x4 socket that Al found on the t610.
I haven't yet checked this but will do so in due course.